1. Field of the Invention
This invention relates to an image information transmission system which transmits image information.
2. Description of the Related Art
The image information transmission systems which have been known include an electronic still video system. The electronic still video system is arranged to record a still image signal, for example, on a magnetic disc which is employed as a recording medium and to reproduce the recorded still image signal from the magnetic disc.
The conventional electronic still video system has been arranged in conformity to a current TV system, such as the NTSC system or the PAL system, and is, therefore, hardly capable of recording or reproducing an image with a high degree of resolution. To solve this problem, a system called a compatible high-definition still video (abbreviated to CHSV) system has been proposed. The CHSV system is arranged, for example, to be capable of recording and reproducing an image signal with the resolution of about 1000 picture elements in the vertical direction and about 1300 picture elements in the horizontal direction for conformity with a high-definition TV system, such as a TV system called a high-vision system.
According to the CHSV system, an image is offset-sampled by means of an image sensor which has, for example, 1000 picture elements in the vertical direction and 1300 picture elements in the horizontal direction. Then, one field amount of an image signal thus obtained by the offset sampling is recorded in one track on the magnetic disc in such a way as to record one picture amount of a high-definition image signal in a total of four tracks.
At the time of reproduction, the high-definition image signal recorded on the magnetic disc is serially reproduced from the tracks one by one. A reproduced signal thus obtained is re-sampled in the same sampling position as the offset sampling performed in recording. Sampling data thus obtained by the re-sampling is reconstructed by temporarily storing the data in an image memory. Then, sampling data that has not been recorded on the magnetic disc is supplemented on the image memory by an interpolation process. In this manner, a high-definition image signal can be recorded and reproduced with the same degree of quality as the high-vision system.
FIG. 1 of the accompanying drawings shows some of effective picture elements included in the image sensing plane on the image sensor. In FIG. 1, marks "O" indicate the picture elements which are to be read out by the offset sampling. Marks "X" indicate the picture elements which are not read out by the offset sampling. Further, in FIG. 1, reference symbols An to Dn denote the respective fields corresponding to the picture elements.
FIG. 2 shows the pattern of tracks formed on the magnetic disc. The signals of four fields An to Dn shown in FIG. 1 are recorded respectively in a form conforming to the format of the electronic still video system. Further, in FIG. 2, a symbol H-SYNC denotes the position of a horizontal synchronizing signal. At the time of reproduction, the signals are reproduced from the magnetic disc. The reproduced signals corresponding to the picture elements marked "O" in FIG. 1 are re-sampled and temporarily stored in an image memory. Then, an interpolation process is carried out on the image memory for the signals corresponding to the picture elements marked "X" in FIG. 1 by using the signals corresponding to the picture elements marked "O".
FIG. 3 shows the amplitude characteristic of the CHSV system. As shown, the amplitude characteristic becomes a point symmetric roll-off characteristic at a Nyquist frequency fNY (fNY represents a frequency which is 1/2 of a sampling frequency fs used at the image sensor shown in FIG. 1). Further, the phase characteristic in the CHSV system must be flat.
Therefore, in the CHSV system, in order to permit making a check for the amplitude and phase characteristics of a transmission path (i.e., a recording/reproduction electromagnetic conversion system) and to obtain a phase reference for sampling, either one vertical interval test (hereinafter referred to as VIT) pulse or one horizontal interval test (hereinafter referred to as HIT) pulse is added to the image signal per vertical blanking period or horizontal blanking period of each field in recording. Further, a pilot signal is multiplexed in recording the image signal on the magnetic disc for a time base variation correcting process (TBC) which is to be performed on the reproduction side of the system.
Incidentally, the width of the VIT pulse or the HIT pulse is 2T (T: 1/fs).
FIG. 4 shows in outline the arrangement of a reproducting apparatus included in the CHSV system. The illustration includes a magnetic disc 51; a motor 52 which is arranged to rotate the magnetic disc 51; a magnetic head 53; a head moving mechanism 54 for moving the magnetic head 53 to an arbitrary position on the magnetic disc 51; a reproduction amplifier 55; a reproduced signal processing circuit 56; a transmission path correction circuit 57 which is arranged to correct changes in signal waveform due to the characteristic of a transmission path; an A/D (analog-to-digital) converter 58; a phase-locked loop (PLL) circuit 59 arranged to generate a sampling clock signal 2fs which has the same time base variations as the time base variations occurring in the image signal, on the basis of a pilot signal for the TBC which is multiplexed with the image signal in recording the image signal; an image memory 60; a system controller 61 which is arranged to control the image memory 60 and also to control the operation of the whole reproducing apparatus; a phase shifter 62 which is arranged to change the phase of the sampling clock signal 2fs generated by the PLL circuit 59 in such a way as to cause the peak of the VIT or HIT pulse to be sampled as will be described later with reference to FIG. 5; an interpolation process circuit 63; a D/A (digital-to-analog) converter 64; and an image signal output terminal 65.
The operation of the reproducing apparatus shown in FIG. 4 is described as follows:
Referring to FIG. 4, when an operation part which is not shown is operated to give an instruction for a reproducing action, for example, on a track "1" shown in FIG. 2, the head moving mechanism 54 moves the magnetic head 53 to the track 1 on the magnetic disc 51. The magnetic head 53 then reproduces a signal recorded in the track 1. The reproduced signal is amplified to a suitable level by the reproduction amplifier 55. The amplified signal is subjected to a frequency demodulation process, a deemphasizing process, etc., at the reproduced signal processing circuit 56. The signal thus processed is supplied to the transmission path correction circuit 57 to be subjected to an amplitude correction process, a phase correction process, etc. After these processes, the signal is supplied to the A/D converter 58.
Meanwhile the reproduced signal outputted from the reproduction amplifier 55 is supplied also to the PLL circuit 59. The PLL circuit 59 extracts a pilot signal for the TBC from the reproduced signal and generates a sampling clock signal 2fs which is phase-locked to the TBC pilot signal. The sampling clock signal 2fs generated by the PLL circuit 59 is supplied to the A/D converter 58 via the phase shifter 62. The A/D converter 58 then digitizes, in synchronism with the sampling clock signal 2fs, the reproduced image signal corrected by the above-stated transmission path correction circuit 57. The digitized image signal is stored in the image memory 60.
Of the sampling data thus stored in the image memory 60, data corresponding to the VIT or HIT pulse is read out by the system controller 61. The system controller 61 looks for the maximum value P0 of the data currently read out and the sample data P-1 and P+1 which are read out before and after the maximum value data P0, and causes the phase shifter 62 to shift the phase of the sampling clock signal 2fs generated by the PLL circuit 59, while causing the digitized data to be stored in the image memory repeatedly, until either a difference between the data P-1 and P+1 reaches a minimum value or the value of the data P0 reaches a maximum value.
When the phase control over the sampling clock signal 2fs is completed and the image signal reproduced from the track 1 shown in FIG. 2 is re-sampled and stored in the image memory 60, the system controller 61 causes the head moving mechanism 54 to move the magnetic head 53 to a track 2 shown in FIG. 2. With the head 53 thus moved to the track 2, the image signal recorded in the track 2 is thus reproduced. Then, in the same manner as described above, the reproduced image signal is re-sampled and stored in the image memory 60. After the track 2, image signals recorded in other tracks 3 and 4 are likewise resampled and stored in the image memory 60.
The image signal which is recorded in the four tracks on the magnetic disc 51 is thus completely resampled and stored in the image memory 60. After that, the interpolation process circuit 63 performs an interpolation process with the data which corresponds to the picture elements marked "O" in FIG. 1 (i.e., data reproduced from the magnetic disc 51 and re-sampled) for the data which corresponds to the picture elements marked "X" in FIG. 1.
After completion of the interpolation process, data stored in the image memory 60 is read out in accordance with an accurate clock signal which has no time base variations and is generated by a reading clock signal generator (not shown). The data thus read out is supplied to the D/A converter 64 to be converted into an analog signal. As a result, a high-definition image signal which is comparable with the so-called high-vision signal is outputted from the output terminal 65.
Further, FIG. 6 shows another arrangement of the reproducing apparatus of the CHSV system. In FIG. 6, the components which are similar to those of the reproducing apparatus of FIG. 4 are indicated by the same reference numerals and the details of them are omitted from the following description. In the case of the reproducing apparatus shown in FIG. 6, a signal reproduced from the magnetic disc 51 is demodulated into an image signal and is stored in the image memory 60. After that, data stored in the image memory 60 is waveform-equalized by an waveform equalizer 66. After the waveform equalizing process, the data is again stored in the image memory 60. The data stored again is supplied to the interpolation process circuit 63 to be subjected to an interpolation process. The interpolated data is supplied to a D/A converter 64 to be converted into an analog signal and is outputted as a high-definition image signal.
In the case of the reproducing apparatus of FIG. 6, the re-sampling clock signal is set at a frequency 2fs which is four times as high as the Nyquist frequency fNY. Therefore, no aliasing takes place in the neighborhood of the Nyquist frequency, so that the waveform equalizing process can be carried out with a high degree of accuracy. It is another advantage of the apparatus of FIG. 6 that the phase shifter of the reproducing apparatus shown in FIG. 4 can be omitted. However, the waveform equalizing process in a digital manner, like in the case of the reproducing apparatus of FIG. 6, has presented the following problems: in a case where a difference between the sample data P0 and the sample data P-1 or P+1is small, i.e., in a case where the current phase of sampling points is greatly deviating from a normal phase of sampling points, as shown in FIG. 7, a digital filter used for the waveform equalizer 66 which is used for waveform equalization must be arranged to have many taps. The digital filter then tends to be affected by noises at signal parts before and after the VIT or HIT pulse. The equalizing accuracy thus would be greatly degraded by noises.